The present invention relates to a mercury sphygmomanometer that is compact, easy to manufacture, and safe from mercury spillage.
A mercury sphygmomanometer is one of several blood pressure-measuring devices currently available. Other measuring devices include aneroid pressure measuring manometers and electronic manometers. Aneroid and electronic manometers tend to be smaller in size, lighter in weight, and more compact than mercury sphygmomanometers, but their accuracy is always in doubt. For most assured accuracy the standard among different blood pressure measuring devices is the mercury sphygmomanometer. The mercury sphygmomanometer measures arterial blood pressure and indicates its measurement by the height of a mercury column. The conventional mercury sphygmomanometer uses a transparent rigid plastic or glass tube through which a mercury column rises and falls during blood pressure measurement. In a typical mercury sphygmomanometer, the mercury tube is tall enough to allow the mercury column to reach a maximum height of 300 mm. Thus, the height of the mercury tube establishes the minimum height or length of the mercury sphygmomanometer when the device is stored or carried.
In order to obviate the size limitation of the conventional mercury sphygmomanometer and to allow a compact size for easy portability and storage, three approaches have been taken. One approach is to use a flexible tube for the mercury column, which could be folded during storage for compactness, e.g. as disclosed in U.S. Pat. No. 2,603,210. Another approach is to provide an articulation or a hinge located part way along the tube and to fold and unfold the mercury tube at the articulation or hinge for storage and usage of the device, e.g. as disclosed in U.S. Pat. Nos. 1,093,199 and 1,077,365. In another approach using a hinge mechanism, as described in U.S. Pat. No. 1,474,853, the mercury column used for blood pressure reading is a single tube. But this device requires a third tube to act as a mercury reservoir which is placed between a tube for the mercury column used for blood pressure readings and another tube connected to the air inflation cuff.
Repeated folding of a flexible tube can damage the tube and cause mercury spillage. Use of a hinge or an articulation would add complexity and cost to the manufacturing process in order to minimize risk of mercury spillage along the hinge or the articulation.
The third type of device is based on reducing the height of the mercury column based on Boyle""s Law, which states: if the volume of a gas and its temperature are kept constant, the pressure of the gas is inversely proportionate to its volume. Unlike a conventional mercury sphygmomanometer with a mercury tube in communication with an outside air, the mercury tube in this device is in a closed system. It is in communication with an air reservoir having a fixed volume through a passageway containing a filter, which allows air to pass through the filter between the two compartments, but blocks mercury from entering the reservoir.
Because the air valve that connects the air reservoir to the outside air is closed, pressure develops in the air reservoir as well as in the air compartment above the mercury column. This air pressure will be exerted on the rising mercury column. Hence, the pressure at the bottom of the mercury column is equal to the air pressure on the top of the mercury column plus the pressure generated by the weight of mercury, which is proportionate to the height of the mercury column. The maximum height of mercury column needed to obtain a certain maximal pressure measurement (e.g. 300 mmHg) can be reduced to a length that is much shorter than 300 mmHg. This feature allows easy portability of the device.
The present invention enables the measurement of blood pressure using a variation of the third principle described above. The invention involves blood pressure measurements using two separate scales that are displayed along the mercury column, e.g. at opposite sides of the column or at one side. One scale, e.g. along one side of the mercury column, represents blood pressure measurements made with the air valve open. The other scale, e.g. along the other side of the mercury column, represents the pressure measurements with the air valve closed. The maximum blood pressure scale with the open air valve is dictated by the desired maximal blood pressure readings. For example, if a maximal blood pressure reading of 180 mmHg is desired, the required height of the mercury column would be 7.086 inches (180/25.4=7.086). If a maximal pressure reading of 200 mmHg is desired, the required maximal height would be 7.874 inches). In either example, the overall length of the device would be considerably shorter than the conventional mercury sphygmomanometer.
The advantage of the dual scale system (one scale, e.g. on one side with a closed air valve, and the other scale, e.g. on the other side with an open air valve) is in the easy readability of the blood pressure scales. When blood pressure is measured entirely with the air valve closed (as claimed by the U.S. Pat. No. xx), the advantage in compactness is counterbalanced by reduced readability of the blood pressure scales. In a conventional mercury sphygomomanometer, the blood pressure scales are depicted alongside the mercury column at intervals of about 2 mm in length. If the device were to be created using the principles claimed by U.S. Pat. No. xxx, the blood pressure scales would have to be depicted at shorter intervals than 2 mm. For example, if the overall length of the mercury column were to be reduced from 300 mmHg to 180 mmHg (reduced to 60%), the actual distance of each 2 mm scale would be only 1.2 mm (reduced to 60%). In the device to be produced using the principle claimed by the present invention, the actual distance of the blood pressure scales with the open air valve would be the same as that of the conventional device. With the open air valve, no additional pressure is exerted on the mercury column, and the pressure at the bottom of the mercury column depends solely on the height of the mercury column. Normotensive subjects and most hypertensive subjects with adequate blood pressure control will only need to use the scale with the open-valve system if the maximal pressure of such device is 180 mmHg. The pressure readings using the device with the open air valve would be as accurate and easy as using the conventional mercury manometer, since the scale of pressure readings are the same as those of a conventional mercury manometer.
When the blood pressure to be measured is higher than the maximal reading possible with the open air valve, measurement would then be repeated with the closed air valve. However, pressure readings made with a closed air valve would not be as accurate as those made with the open air valve for two reasons. 1) The air pressure that develops on the top of the mercury column may vary with variations in the atmospheric pressure. 2) Cramming of the displayed pressure scales into a shorter height space makes it harder to read the scales. However, A slight error in readings with the closed air valve is clinically unimportant, because the closed air valve scale would be used only when the pressure is higher than the maximal readings possible with the open air valve scale; when the pressure is lower, the open air valve scale is used. For example, a distinction between a pressure difference of 85 and 90 mmHg would be more important while the distinction between 250 and 255 mmHg is likely less important.
A second feature of the present invention relates to minimizing errors in pressure measurement resulting from variation in atmospheric pressure due primarily to a change in the altitude at which the measurement is made. When blood pressure is measured using the closed air valve, the air in the mercury tube above the mercury column is compressed within a closed system that communicates with the air reservoir when the mercury column rises. The rise in pressure in the air reservoir system during a blood pressure measurement depends on the magnitude of the reduction in the air space caused by a rising mercury column in relation to its overall original size. Another important fact is the original pressure in the air reservoir, which is equal to the atmospheric pressure where the device is used. For example, if the device is used at a sea level, reduction in air space volume from 7.33 ml to 6.33 ml, would increase the pressure to 880 mmHg (an increase of 120 mmHg) as shown in the following equation: 760xc3x977.33=880xc3x976.33.
Barometric pressure is not constant, but a daily variation in any given area is usually less than 5 mmHg. Over the past 150 years, the maximal variation in pressure was shown to be about 15 mmHg. This magnitude of variations in atmospheric pressure would have little effect on the overall blood pressure calculation. An error in blood pressure reading at 100 mmHg level with the variation in atmospheric pressure by 5 mmHg, would be less than 1 mmHg.
Much larger changes in the atmospheric pressure occur with a change in altitude than with a change in weather. For example the atmospheric pressure at an altitude of 5,000 feet (Denver, Colo.) is about 632 mmHg (a reduction of 128 mmHg). See Table 1 which appears as FIG. 2 hereof.
A blood pressure measurement based on an assumed atmospheric pressure of 760 mmHg in an area with an actual atmospheric pressure of 632 mmHg would result in a substantial error in the blood pressure measurement. For example, if the same device as above were used with the air valve closed at an altitude of 5,000 feet (632 mmHg of atmospheric pressure), a reduction in air space volume from 7.33 to 6.33 ml would increase the pressure from 632 to 732 mmHg (increase of only 100 mmHg). To minimize an error caused by variations in atmospheric pressure due to changes in altitude, solid chips of known volume may be placed into the air reservoir chamber or another device may be attached or placed in the air reservoir which can be manually adjusted to vary the volume of air in the air reservoir. This can be achieved by reducing the air reservoir volume. Table 2 which appears as FIG. 3 hereof shows a sample calculation for the number of chips of one size that might be added to adjust air reservoir volume at different altitudes in order to compensate for reduced atmospheric pressure. With these adjustments, the error in pressure readings may be 2 mmHg or less at an altitude of 10,000 feet. At lower altitudes, errors are much lower. These magnitudes of errors are clinically insignificant, especially because the errors may occur only in a closed system which is used only when blood pressure value is very high. Changes in the volume of the air reservoir can be made when the user of the device moves to a different location. The information on altitude of different cities or localities could be shown in the instruction manual of the device along with instructions on compensating a closed system.
In a third aspect of the present invention, a mechanism is provided that measures the atmospheric pressure (and therefore the altitude indirectly) of the location where the device is being used. This would be accomplished by creating a mark (or marks) alongside the blood pressure scale that would coincide with the mercury drop level during the mercury drop test described below.
Mercury drop test: Fill the mercury tube to the top with mercury by tilting the device while the air reservoir valve is open. (There should be no air bubble in the mercury column during this procedure). When the mercury column reaches the top of the mercury tube, it will not advance any further because the top is blocked by a filter that allows air passage, but not mercury passage. At this point, close the air valve, and then let the device stand in its normal position. As the mercury level falls under the force of gravity, a negative pressure develops in the air reservoir because the falling mercury level creates additional space above the mercury column. When the negative pressure in the air reservoir equals the pressure generated by the height of the mercury column, the mercury level stops falling.
The amount of negative pressure that develops with the falling mercury level during the mercury drop test depends on the baseline air pressure of the reservoir, which is equal to the atmospheric pressure when the air valve is open. At sea level (an atmospheric pressure of 760 mmHg), the mercury level drop can be calculated using the following formula:
760xc3x976.33=Pxc3x97(6.33+A/180), where P is the new pressure inside the air chamber after the mercury drop (which is subatmospheric pressure), 760 is the atmospheric pressure at the sea level, and A the mercury level at the top of the mercury column depicted on the scale of 0 to 180.
The difference between 760 and P is the magnitude of negative pressure in the air reservoir, and this would be equal to the A. A calculation using the above formula at sea level indicates that mercury would drop to the level of 68 mmHg during the mercury drop test. If the same test were performed at an altitude of 5,000 feet (the atmospheric pressure of 632 mmHg), the mercury level during the drop test would produce a level of 60.3 mmHg. The drop is greater because the magnitude of negative air pressure that develops in the air reservoir by the mercury drop would be less because of the lower baseline atmospheric pressure. The depiction of such a scale alongside the mercury column can be used to determine the atmospheric pressure and altitude of the place where the device is used. When the mercury drop test is repeated after reduction of the air reservoir volume e.g. by addition of an appropriate number of chips, the mercury level would drop to the level similar (67 mmHg) to that at sea level.
Table 3 appears as FIG. 4 hereof and shows the mercury level drop at various altitudes and the drop after correction of air reservoir volume.